A major, maybe extremely kamikaze, back in army (3)
I believe the answer is:
key
'a major maybe' is the definition.
(A major is a musical key)
'extremely kamikaze back in army' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'back in' indicates one should take the final letters (the 'back' of the word).
'kamikaze' with its middle taken out is 'ke'.
The final letter of 'army' is 'y'.
'ke'+'y'='KEY'
(Other definitions for key that I've seen before include "//www.crosswordgiant.com/crossword-clue/1252911/" , "It can open" , "Crucially important" , "It opens the door" , "Typewriter button, isle" .)